In wireless power transmission for performing contactless and noncontact power transmission, there is invented a wireless power transmission system that performs an authentication process between a power feeding device and a power receiving device and starts a power feeding after completing the authentication (see Japanese Patent Application Laid-Open No. 2008-295191, for example). According to a technique disclosed in Japanese Patent Application Laid-Open No. 2008-295191, there is no guarantee that after a power feeding is started, power supplied from the power feeding device to the authenticated power receiving device is continuously supplied.
Particularly, when a physical distance between the power feeding device and the power receiving device increases, there is a risk that other device receives power from the power feeding device. Further, there is also a risk that other device steals power.